White light emitting apparatus and line illuminator using the same in image reading apparatus

ABSTRACT

A white light emitting apparatus includes a light source section in which a first white LED that emits white light whose chromaticity deviates to a blue side from a predetermined white region of a CIE chromaticity diagram and a second white LED that emits white light whose chromaticity deviates to a yellow side from the predetermined white region are adjacently disposed so as to emit light with optical axes in substantially the same direction, and a current regulator that independently drives the blue LED chip in the first and second white LEDs, respectively. A color mixture of lights emitted from the first and second white LEDs is adjusted to a chromaticity of the predetermined white region using the current regulator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part application of International ApplicationNo. PCT/JP2008/063839, with an international filing date of Aug. 1,2008, which designating the United States of America, the entirecontents of which are incorporated herein by reference.

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2008-031470, filed on Feb. 13,2008, the Japanese Patent Application No. 2009-186880, filed on Aug. 11,2009, and the Japanese Patent Application No. 2010-049662, filed on Mar.5, 2010, the entire contents of which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a white light emitting apparatus and aline illuminator that uses the white light emitting apparatus in animage reading apparatus. In particular, the present invention relates toa white light emitting apparatus on which a white LED is mounted and aline illuminator that uses the white light emitting apparatus, that areused in an image reading apparatus.

BACKGROUND ART

In a machine such as a fax machine, a copier, or a hand scanner, animage reading apparatus such as an image sensor is used as an apparatusfor reading an original copy. Normally, a contact-type image sensor thathas a short optical path length and that can be easily incorporated intothe machine is used as this kind of image reading apparatus.

A contact-type image sensor is equipped with a line illuminator thatlinearly illuminates an original copy surface across a main scanningarea. As a line illuminator, an indirect illumination-type lineilluminator is known that uses a long light guide member and thatpropagates a light incident from a light source disposed at an endsurface of the light guide member while causing the light to bereflected by an inner surface to emit the light from a linear emittingsurface in a longitudinal direction.

It is known that a white LED that includes a blue LED chip and a YAGphosphor can be used as a light source in this kind of line illuminator.A white LED light source is capable of reading color images, andlighting control thereof is simpler in comparison to a multi-chip LEDthat emits light of the three colors red, green and blue (RGB). Further,it is said that a white LED light source is superior in terms ofbrightness characteristics and with respect to miniaturization andweight reduction, and also enhances light emission efficiency whenemployed as the light source of a contact-type image sensor.

Further, users desire that in a line illuminator in which an LEDincluding a plurality of light emitting elements is disposed as a lightsource at an end surface of a light guide member, emitted light colorsfrom the plurality of chips are released after being sufficiently mixedin the light guide member without disturbing the color balance, andilluminate a color original copy as linear illumination with ahomogeneous hue. An invention for solving these problems or that has afeature relating to the shape of a light guide member is disclosed inPatent Document 1 and the other documents.

Further, Patent Document 2 discloses that when white LEDs are massproduced, considerable variations arise with respect to the lightemission wavelength and light emission brightness of blue LED chips.Furthermore, white light that is mixed is also affected by variations inthe amount and dispersion of fluorescent particles of YAG phosphor thatget mixed in with a covering member and the like, and it is consideredthat, as a result, large variations arise in the hue and brightness ofcompleted white LEDs.

The distribution of variations in the hue of emitted lights of whiteLEDS that were mass produced in lots will now be described referring toFIG. 16 using one section of the chromaticity coordinates thereof. Thedistribution chart shown in FIG. 16 is disclosed in Patent Document 2.Each black spot illustrated in FIG. 16 represent individual hue data ofa white LED, and the hue is dispersed in a band shape that slopes upwardto the right as illustrated in FIG. 16. More specifically, the hue isdispersed in a band shape in a white region that falls approximately ona line that passes from blue through yellow via a white point accordingto the chromaticity classification of JIS Z 8110. In this case, thedispersion in the width direction (represented by the arrowed line A) ismainly due to hue variations caused by variations in the light emissionwavelength of blue light emitting elements, and the dispersion in thelongitudinal direction (represented by the arrowed line B) is mainly dueto hue variations caused by variations in the amount or dispersion offluorescent particles that have gotten mixed into the covering member.In this connection, since the light emission wavelength of blue lightemitting elements varies significantly between lots, the dispersion inthe width direction (represented by the arrowed line A) spreads furtherin actual mass production (see Patent Document 2).

In this case, when attempting to construct a light source of a lineilluminator for an image reading apparatus using such kind of white LED,it is preferable from the viewpoint of the central hue and hueunevenness and the like that the light source is constructed using onlywhite LEDs that have been selected by ranking the emitted light colorsthereof in a desired chromaticity range. However, when a light source isconstructed using only white LEDs that have a specific ranking, whiteLEDs with a different ranking are wasted, and there is thus the problemthat the total cost for obtaining the white LEDs is comparatively high.

-   [Patent Document 1] Japanese Laid-Open Patent Publication No.    2006-287923-   [Patent Document 2] Japanese Laid-Open Patent Publication No.    2004-119743-   [Patent Document 3] Japanese Patent No. 3990437-   [Patent Document 4] Japanese Laid-Open Patent Publication No.    2002-134284-   [Patent Document 5] Japanese Laid-Open Patent Publication No.    2007-066969-   [Patent Document 6] Japanese Laid-Open Patent Publication No.    2006-303373

SUMMARY OF THE INVENTION

The present invention has been made in view of the above describedconventional problems, and provides a white light emitting apparatusconstructed by combining white LEDs that emit lights with degrees ofwhiteness over a wide range that deviate from a desired degree ofwhiteness, and that mixes the emitted light colors to emit light with adesired chromaticity, as well as a line illuminator that uses the whitelight emitting apparatus. An object of the present invention is toimprove yields of white LEDs and lower the cost of producing a whitelight emitting apparatus by increasing the usability of white LEDs thathave a wide range of chromaticities.

A white light emitting apparatus for an image reading apparatusaccording to the present invention uses a white LED having a blue LEDchip and a phosphor, and includes a light source section in which afirst white LED that emits white light whose chromaticity deviates to ablue side from a predetermined white region of a CIE chromaticitydiagram and a second white LED that emits white light whose chromaticitydeviates to a yellow side from the predetermined white region areadjacently disposed such that optical axes thereof are in substantiallythe same direction, and a current regulator that independently drivesthe blue LED chips in the first and second white LEDs, respectively;wherein the current regulator performs PWM control for a drive currentof at least one of the first and second white LEDs, and by settingrespective pulse widths for driving the first and second white LEDs thatare inversely proportional to a distance from a target chromaticitypoint to a chromaticity point of emitted light of the first white LEDand a distance from the target chromaticity point to a chromaticitypoint of emitted light of the second white LED on a CIE chromaticitydiagram, adjusts a color mixture of lights emitted from the first andsecond white LEDs to a chromaticity of the predetermined white region.

A white light emitting apparatus for an image reading apparatusaccording to the present invention uses a white LED having a blue LEDchip and a phosphor, and includes a light source section in which afirst white LED that emits white light whose chromaticity deviates to ablue side from a predetermined white region of a CIE chromaticitydiagram and a second white LED that emits white light whose chromaticitydeviates to a yellow side from the predetermined white region areadjacently disposed such that optical axes thereof are in substantiallythe same direction, and a current regulator that independently drivesthe blue LED chips in the first and second white LEDs, respectively;wherein the current regulator continuously supplies a constant currentto one white LED among the first and second white LEDs that emits whitelight whose chromaticity is nearer to a target chromaticity point on theCIE chromaticity diagram, and supplies a PWM-controlled current toanother white LED among the first and second white LEDs, sets a dutyratio of the PWM control to a value of an inverse ratio between adistance between a chromaticity of the one white LED and the targetchromaticity point and a distance between a chromaticity of the otherwhite LED and the target chromaticity point on the CIE chromaticitydiagram, and adjusts a color mixture of emitted lights from the firstand second white LEDs to a chromaticity of the predetermined whiteregion.

A line illuminator according to the present invention is configured toemit light that is incident from a light source disposed toward a lightincident surface provided at an end surface in a length direction of abar-shaped light guide member formed of a clear member from a lightemitting surface provided along a length direction while causing thelight to be reflected by an inner surface of the bar-shaped light guidemember. The light source is the above described white light emittingapparatus.

According to the line illuminator of the present invention, an externalshape of a radiating surface that radiates a light of the white lightemitting apparatus is of dimensions that can be included in an externalshape of the light incident surface of the light guide member.

According to the white light emitting apparatus of the presentinvention, by using a combination of white LEDs that emit light colorswhose chromaticity deviates from a predetermined white region amongranked white regions that are being provided as products, it is possibleto utilize white LEDs that have a wide range of chromaticities. Morespecifically, the white light emitting apparatus of the presentinvention lowers the cost of producing white LEDs by reducing the numberof products that have heretofore been wasted due to hue deviations thatare inherent to white LEDs that include a blue LED chip and a YAGphosphor.

Further, according to the white light emitting apparatus of the presentinvention, it is possible to emit light of a predetermined white regionthat is a target without employing a complicated configuration that hasbeen used in the conventional technology such as newly adding a LED chipthat emits light of a different wavelength or a different kind offluorescent substance, or changing the phosphor composition in order tocompensate for chromaticity variations that are a drawback of whiteLEDs.

Further, for example, by using the white light emitting apparatusaccording to the present invention as a light source of a lineilluminator, a bright line illuminator that illuminates with aprescribed white chromaticity can be economically produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective diagram illustrating a light source section of awhite light emitting apparatus according to a First Embodiment of thepresent invention;

FIG. 2 is a diagram illustrating one example of an electrical circuitfor driving the white light emitting apparatus according to the FirstEmbodiment of the present invention;

FIG. 3 is a diagram for describing control of the illumination durationof two white LEDs according to the First Embodiment of the presentinvention;

FIG. 4 is a view for describing a typical white region usingchromaticity coordinates according to JIS Z 8110;

FIG. 5A is a view that illustrates an example in which light emissionregions of white LEDs produced by one manufacturer are divided into fourranks and represented on a CIE chromaticity diagram;

FIG. 5B is a view that illustrates coordinate values of angles of eacharea generated by the division of light emission regions into ranksillustrated in FIG. 5A;

FIG. 6A is a view that illustrates an example in which light emissionregions of white LEDs produced by one manufacturer are divided intoseven ranks and represented on a CIE chromaticity diagram;

FIG. 6B is a view that illustrates coordinate values of angles of eacharea generated by the division of light emission regions into ranksillustrated in FIG. 6A;

FIG. 7A is a view in which the chromaticity of emitted lights ofcommercially available white LED products are re-ranked into areas A, B,and C and represented on a CIE chromaticity diagram;

FIG. 7B is a view that illustrates coordinate values of each angle ofthe areas A, B, and C that have been re-ranked as illustrated in FIG.7A;

FIG. 8 is a chromaticity diagram that describes control that mixescolors of lights emitted from two white LEDs of the white light emittingapparatus according to the First Embodiment of the present invention;

FIG. 9 is a view for describing a white light emitting apparatus used asa first or second white LED that emits light with a chromaticity in aband region that is outside a ranked white region;

FIG. 10 is a view for describing the chromaticity of light emitted by awhite light emitting apparatus of a Second Embodiment according to thepresent invention;

FIG. 11 is a diagram for describing control of the illumination durationof two white LEDs of the Second Embodiment according to the presentinvention;

FIG. 12 is a perspective diagram illustrating the configuration of aline illuminator of a Third Embodiment according to the presentinvention;

FIG. 13 is a view for describing a radiating surface of a white lightemitting apparatus used for a light source of the line illuminator ofthe Third Embodiment according to the present invention;

FIG. 14 is a view that illustrates a cross section of a CIS unit thatincorporates the line illuminator of the Third Embodiment according tothe present invention;

FIG. 15 is a view for describing an illuminance distribution in alongitudinal direction when an illumination light of a line illuminatorof the Third Embodiment according to the present invention is dividedinto red (R), green (G), and blue (B); and

FIG. 16 is a distribution chart that illustrates variations in thechromaticity of white LEDs.

FIG. 17 is a perspective diagram illustrating a light source section ofa white light emitting apparatus according to a fourth embodiment of thepresent invention;

FIG. 18 is a diagram illustrating one example of an electrical circuitdriving the white light emitting apparatus according to the fourthembodiment of the present invention;

FIG. 19 is a chromaticity diagram illustrating a relationship betweenchromaticities of emitted lights from the white light emitting apparatusaccording to the fourth embodiment of the present invention;

FIG. 20 is a diagram illustrating a control of illumination duration ofa white light emitting diode in the fourth embodiment of the presentinvention;

FIG. 21 is a cross sectional diagram illustrating a modified embodimentof the white light emitting apparatus according to the fourth embodimentof the present invention;

FIG. 22 is a perspective diagram illustrating a linear illuminatoraccording to a fifth embodiment of the present invention;

FIG. 23 is a cross sectional diagram illustrating a contact-type imagesensor unit in which the linear illuminator according to the fifthembodiment of the present invention is incorporated;

FIG. 24A is a diagram illustrating the relative illuminance between eachof R, G, and B colors before an adjustment of illumination duration inthe linear illuminator according to the fifth embodiment of the presentinvention;

FIG. 24B is a diagram illustrating the relative illuminance between eachof R, G, and B colors after the adjustment of illumination duration inthe linear illuminator according to the fifth embodiment of the presentinvention;

FIG. 25A is a diagram illustrating a method for manufacturing a whitelight emitting diode according to a sixth embodiment of the presentinvention (a configuration before a fluorescent layer is formed);

FIG. 25B is a diagram illustrating the method subsequent to FIG. 25A formanufacturing a white light emitting diode according to the sixthembodiment of the present invention (a configuration after a fluorescentlayer is formed);

FIG. 26 is a diagram illustrating a state with the white light emittingdiode according to the sixth embodiment of the present invention beingmounted to a printed circuit board;

FIG. 27 is a chromaticity diagram illustrating a relationship betweenchromaticity of emitted lights from the white light emitting diodeaccording to the sixth embodiment t of the present invention;

FIG. 28 is a perspective diagram illustrating a linear illuminatoraccording to a seventh embodiment of the present invention;

FIG. 29 is a cross sectional diagram illustrating a contact-type imagesensor unit in which the linear illuminator according to the seventhembodiment of the present invention is incorporated;

FIG. 30 is a chromaticity diagram illustrating a relationship betweenthe degrees of whiteness of a white light emitting diode andidentification areas.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention was completed based on the study of properties ofa white LED having a blue LED chip and a fluorescent substance by theinventor of the present application, the study of a white light emittingapparatus and an illuminator for emitting white light that emit ahigh-power light having a uniform degree of whiteness, and a novelfinding of a configuration of these which is easy to manufacture.

In this connection, white colors as illumination light sources in thepresent embodiment are typically defined as in reference FIG. 1 “GeneralChromaticity Classification of System Color Names” of JIS Z 8110 in theJapanese Industrial Standards as illustrated in FIG. 4. Among these, thewhite colors are classified into white, (bluish) white, (purplish)white, (yellowish) white, (greenish) white and (light) pink. Thesecolors are referred to as a typical “white color” according to thepresent invention.

A process whereby a mixture ratio of light is adjusted so that colors ofillumination light sources become a target white color is referred to as“white balance”. This adjustment is performed using a measurement sensordevice. According to the present embodiment a spectrophotometer is used,and the luminosity, brightness, chromaticity, and x and y coordinates ona CIE chromaticity diagram and the like of a white LED is measured orcalculated. However, the present invention may also be implementedsatisfactorily even when the white balance is adjusted using ameasurement sensor device other than a spectrophotometer.

Each manufacturer of white LEDs ranks regions of a certain width andlength in the aforementioned typical white color region including awhite point (x=0.33, y=0.33) on the CIE chromaticity diagram as lightemission regions of white LEDs that are provided. For example, in theproduct catalog (fiscal year 2008 edition) of Nichia Corporation(hereunder abbreviated to “N. Corp.”), light emission regions are rankedand divided into four regions, namely, a0, b1, b2, and c0, asillustrated in FIG. 5A. The coordinate values of angles of each regionare illustrated in FIG. 5B. Further, in the product catalog (2008edition) for white LEDs of the type made by combining blue LEDs andphosphor of Toyoda Gosei Co. Ltd. (hereunder, abbreviated to “T. Co.”),as illustrated in FIG. 6A, light emission regions are ranked and dividedinto seven regions, namely, AA, AB, B3 to B6, and C0. The coordinatevalues of angles of each region are illustrated in FIG. 6B. The totalregion ranked by both companies is identical and overlapping on the CIEchromaticity diagram, and the respective regions only differ at thelevel of the segmented regions.

Here, to facilitate the description of the present embodiment, theseranked regions are re-ranked into three regions, namely, A, B, and Cregions. More specifically, the a0 region of N. Corp. and the (AA+AB)region of T. Co. are the same region (hereunder, referred to as “Aregion”), the c0 region of N. Corp. and the CO region of T. Co. are thesame region (hereunder, referred to as “C region”), and the (b1+b2)region of N. Corp. and the (B3+B4+B5+B6) region of T. Co. are the sameregion (hereunder, referred to as “B region”), and are overlappingregions, respectively. The light emission regions re-ranked in thismanner are illustrated in FIG. 7A. The A region, B region, and C regionillustrated in FIG. 7A are regions for which the chromaticity of emittedlights of commercially available white LED products are ranked. Thecoordinate values of angles of each region are illustrated in FIG. 7B.According to FIG. 7 A, the white point (x=0.33, y=0.33) exists on theboundary between the B region and C region. As illustrated in FIG. 7A,the B region is a region in which the color changes towards blue fromthe white point.

Hereunder, the (A+B+C) region is referred to as a “ranked white region”.Further, the B region is referred to as a “predetermined white region”(desired hue), and is the target hue of an emitted light color of thewhite light emitting apparatus in the embodiment. Thus, with respect tothe ranked white region for which manufacturers have ranked the whitelight emission of white LEDs, the predetermined white region (B region)is positioned in the centre thereof, and the A region that deviates tothe blue side and the C region that deviates to the yellow side from thepredetermined white region are regions on the CIE chromaticity diagramthat are connected to the predetermined white region (B region). Asillustrated in FIG. 7B, the predetermined white region (B region)represents chromaticities inside a region surrounded by four points withthe coordinate values (x, y) (0.296, 0.276), (0.283, 0.305), (0.330,0.360) and (0.330, 0.318) on the CIE chromaticity diagram. Further, theA region represents chromaticities inside a region surrounded by thefour points (0.280, 0.248), (0.264, 0.267), (0.283, 0.305), and (0.296,0.276), and the C region represents chromaticities inside a regionsurrounded by the four points (0.330, 0.318), (0.330, 0.360), (0.361,0.385) and (0.356, 0.351).

In an image reading apparatus in which the white light emittingapparatus or line illuminator according to the present invention isused, it is not intended to have the light source or reflected lightthereof directly recognized by the human eye, and a mechanism isemployed whereby a photoelectric conversion sensor receives andrecognizes the light. Accordingly, it is not necessary that the centralhue of emitted light colors must always be the white point (x=0.33,y=0.33) on the CIE chromaticity diagram, and the hue of the light sourceis selected by taking into account the cost efficiency of the lightsource in an adjustable range of the image reading apparatus.

In this connection, the coordinate values (x=0.307, y=0.315) of a commonangle of the regions B3 to B6 shown in FIG. 6A are employed as thecoordinates of the center of the B region, and are taken as targetcoordinate values (target chromaticity point) which the chromaticityshould reach in the present embodiment. Accordingly, althoughconstructing a light source of an image reading apparatus that is anobject of the present invention using only white LEDs of B region ispreferable from the viewpoint that hue unevenness is easily suppressed,in such case white LEDS of the A region and C region are wasted, andthis is a problem should be considered from the cost viewpoint.

The present invention provides a white light emitting apparatus or aline illuminator that, with respect to white LEDs that emit light of achromaticity of a ranked white region provided by a manufacturer, emitslight using white LEDs that emit light colors of an A region or a Cregion that deviate from a predetermined white region (B region), andadjusts the hues of the emitted light colors. As a result, the number ofwasted white LEDs is reduced. Specific examples of the invention aredescribed hereunder.

First Embodiment

A white light emitting apparatus used in an image reading apparatusaccording to the First Embodiment is provided with two white LEDs. Eachof the white LEDs includes a blue LED chip, and a phosphor layer whichis excited by a radiation light emitted by the blue LED chip and emits ayellow light. Yellow is a complementary color of blue. In the FirstEmbodiment, a white light emitting apparatus 20 having two white LEDswill be explained with reference to FIGS. 1 to 4, and FIG. 9.

The white light emitting apparatus 20 of the present embodiment includesa light source section 10 and a current regulation section 33. FIG. 1illustrates the light source section 10 of the white light emittingapparatus 20. The light source section 10 includes a first white LED 11for emitting a white light of A region which deviates to the blue sidefrom the predetermined white region (B region) among the aforementionedranked white region (A region+B region+C region) and a second white LEDfor emitting a white light of C region which deviates to the yellow sidefrom the predetermined white region (B region), with the first andsecond LEDs 11 and 12 being adjacently mounted to a printed circuitboard 15. The first and second white LEDs 11 and 12 are also adjacentlyarranged to emit light such that the principal light-emitting directionsthereof, that is, the optical axes, are parallel to each other inapproximately the same direction. The printed circuit board 15 isprovided with an anode line which is common to the two white LEDs 11 and12 and two cathode lines connected to each of the white LEDs 11 and 12as wiring 16 for electric supply. The anode line and the two cathodelines are connected to an external current regulation section 33 throughterminals A, K1, and K2, respectively (see FIG. 2). The currentregulation section 33 causes the first and second white LEDs 11 and 12constituting the light source section 10 of the white light emittingapparatus 20 to be driven independently. In this connection, thephosphor layer covering the blue LED chip is omitted from FIG. 1.

As the white LEDs used in this embodiment, for example, white LEDs forwhich the hues of emitted lights are in the A region and the C regionmay be designated and acquired. Each of these kinds of white LED is acommercially available surface mount LED package having a longitudinaldimension of about 2.0 mm and a lateral dimension of about 1.2 mm(NESW007A, manufactured by Nichia Corporation). Further, it is possibleto acquire the white LEDs with the aforementioned product number and,from among those, identify and use white LEDs that emit light of the Aregion and white LEDs that emit light of the C region in a lightingstate with a rated forward current of 10 mA by means of theaforementioned measurement sensor device.

The white light emitting apparatus according to the present invention isan apparatus that attempts to obtain the chromaticity of B region bymixing the white lights of two white LEDs. That relationship isdescribed next referring to a CIE chromaticity diagram illustrated inFIG. 8. The coordinate values of angles of each region are the same asin FIG. 7B. In this case, the white light emitting apparatus includes afirst white LED that emits a white light LA1 at a chromaticity point PA1in A region on the CIE chromaticity diagram illustrated in FIG. 8, and asecond white LED that emits a white light LC1 at a chromaticity pointPC1 in C region illustrated in FIG. 8.

In order to cause a color mixture produced by the emitted light colorsof the first and second white LEDs to match a point PD0 illustrated inFIG. 8, the luminosity of the white light LA1 at the point PA1 and theluminosity of the white light LC1 at the point PC1 may be weighted, andthe luminosity may be adjusted so that a value that is a weightedaverage between the chromaticity coordinates of the point PA1 and thechromaticity coordinates of the point PC1 becomes a value withpredetermined chromaticity coordinates. The point PD0 is a target pointwhich the color mixture should reach that is taken to have thecoordinate values (x=0.307, y=0.315), and these coordinates are thecenter of the aforementioned B region.

Adjustment of the luminosity (brightness) of emitted light colors of thewhite LEDs of the present embodiment is performed with PWM control thatkeeps a forward current that drives each LED chip constant and changesthe pulse width, which is recommended because there are few hue changes.

Next, a method of adjusting a color mixture in the white light emittingapparatus 20 of the present embodiment is described. FIG. 2 is a diagramillustrating one example of an electrical circuit for driving the whitelight emitting apparatus 20. The electrical circuit includes the lightsource section 10 and a current regulation section 33 that is a currentregulator. The light source section 10 corresponds to the partillustrated in FIG. 1. The current regulation section 33 includescurrent regulation sections that are separately provided to controlcurrents flowing to each of the first and second white LEDs 11 and 12.For example, current regulating circuits 21 and 22 are connected inparallel to each of the first and second white LEDs 11 and 12 via twocathode terminals K1 and K2 of the light source section 10,respectively. Also, transistors T1 and T2 for turning each of the whiteLEDs 11 and 12 on and off are connected to the current regulatingcircuits 21 and 22, respectively. The transistors T1 and T2 areconnected to ground GND.

Each of the current regulating circuits 21 and 22 are, for example,provided with an operation amplifier, a transistor, and acurrent-limiting resistor R1 or R2. The current regulation section 33causes a forward current that is defined for each of the white LEDs 11and 12 to be applied to each of the white LEDs 11 and 12, and turns thetransistors T1 and T2 on and off, respectively, by a PWM control methodso as to control the lighting of the white LEDs 11 and 12. Such currentregulation section 33 functions as a current regulator.

Next, the current control operation for causing emitted light colorsthat have been mixed by the white light emitting apparatus 20 of thepresent embodiment to substantially match a target point that should bereached on a chromaticity diagram will be described. FIG. 3 is a timingchart for performing PWM control with respect to an illuminationduration of the first and second white LEDs 11 and 12.

First, the white light emitting apparatus 20 is operated with a durationof pulse T being set to be 10 milliseconds, a current for driving thefirst and second white LEDs 11 and 12 being set to be a rated 10 mAusing the current regulating circuits 21 and 22, and an illuminationduration t1 of the first white LED 11 being set to be 5 milliseconds foreach cycle. The drive currents of the white LEDs 11 and 12 are set bythe current regulating circuits 21 and 22, respectively, and theillumination duration t1 is controlled by the transistor T1.

Next, a measurement sensor device is installed at a position above thelight emitting surface of the white light emitting apparatus 20 that isseparated from the two white LEDs 11 and 12 by a distance such that theemitted lights are sufficiently mixed with each other, or over ascattering plate that has been arranged, and chromaticity measurement ofemitted light colors of the white light emitting apparatus 20 isstarted. In this case, chromaticity measurement is started with ameasurement time being set to be several tens of times or more greaterthan the duration T.

Thereafter, an illumination duration t2 of the white LED 12 for eachcycle is controlled by the transistor T2, to find an illuminationduration t2 for which a chromaticity measurement value of light emittedby the white light emitting apparatus 20 approximately matches the pointPD0 (x=0.307, y=0.315) at the center of the B region illustrated in FIG.8.

In this case, in order to make the chromaticity of an emitted lightcolor that has been mixed by the white light emitting apparatus 20approximately match the point PD0 that has coordinate values atapproximately the center of the B region that is taken as the target onthe CIE chromaticity diagram, the PWM control by the current regulationsection 33 may be used to cause the first white LED 11 to light inaccordance with a duty ratio D1=t1/T and cause the second white LED 12to light in accordance with a duty ratio D2=t2/T.

In this connection, depending on the coordinates position inside the Aregion corresponding to the emitted light color of the first white LEDor the coordinates position inside the C region corresponding to theemitted light color of the second white LED that have been selected, itmay not be possible to match the color mixture with the point PD0.However, since it is possible to match the color mixture with achromaticity inside the B region, it is possible to satisfactorily solvethe problem to be solved by the present invention. Further, the entireranked white region includes the A region+B region+C region that areconnected in an arch shape. Hence, when a white LED is selected thatemits light whose chromaticity is on the inner side of a far end of thearch shape in the A region or C region, in some extremely rare cases thechromaticity coordinates of the color mixture may deviate outside the Bregion. In such case, it is sufficient to determine whether thechromaticity of the color mixture is allowable or not.

In contrast to the conventional white light emitting apparatus thatsuppresses the occurrence of color unevenness by selecting only whiteLEDs that emit light of a predetermined white region that iscomparatively narrow, the white light emitting apparatus according tothe present invention also combines and uses white LEDs that emit lightcolors whose chromaticity deviates from a predetermined white region inranked white regions that are being provided as products. Consequently,according to the present invention, the product cost can be lowered byreducing the number of white LEDs that heretofore have been screened andnot used due to hue deviations that are inherent to white LEDs thatinclude a blue LED chip and a YAG phosphor.

Further, according to the present invention, it is possible tomanufacture a white light emitting apparatus that can emit light of atarget predetermined white region without employing a complicatedconfiguration that has been used in the conventional technology such asnewly adding an LED chip that emits light of a different wavelength or adifferent kind of fluorescent substance, or changing the phosphorcomposition in order to compensate for chromaticity variations that area drawback of white LEDs.

As the current control in the white light emitting apparatus 20, insteadof the control with the PWM method, the amounts of currents for drivingthe white LEDs 11 and 12 may be controlled. The amounts of currents canbe controlled by the current regulating circuits 21 and 22.Alternatively, the control with the PWM method and the control of thecurrent amounts may be combined.

According to the above embodiment, the chromaticity of light emitted bywhite LEDs is described based on ranked white regions provided bymanufacturers. However, with respect to the distribution of thechromaticity of light emitted from white LEDs including a blue LED chipand a YAG phosphor, the chromaticity can also be distributed in a whiteregion between a light emission region of blue of a blue LED chip and ayellow region of light emitted by a phosphor on the CIE chromaticitydiagram. This white region is represented by a band region 66illustrated in FIG. 9. The typical white region 67 according toreference FIG. 1 (see FIG. 9) of the standard JIS Z 8110 that ispreviously defined spreads to outside the ranked white region (Aregion+B region+C region) 65, and is an ellipsoidal region with alongitudinal diameter that is approximately a distance from a point(x=0.23, y=0.21) on a blue side with respect to the white point in theCIE chromaticity diagram to a point (x=0.41, y=0.41) on a yellow sidethereof. The present invention can also be used for a white LED forwhich a chromaticity distributed by the emitted light color of the whiteLED is in the band region 66 within the typical white region 67.

A white light emitting apparatus is manufactured that is adapted in asimilar manner to the aforementioned embodiment by employing a white LEDthat emits light of a white color whose chromaticity deviates to theblue side from the desired white region (B region) in the band region 66on the outside of the ranked white region as a first white LED, andlikewise employing a white LED that emits light of a white color whosechromaticity deviates to the yellow side from the desired chromaticityregion (B region) in the band region 66 on the outside of the rankedwhite region as a second white LED. Even if the emitted light color ofthe first white LED has a chromaticity in a white region that deviatesfurther to the blue side than the A region, or even if the emitted lightcolor of the second white LED has a chromaticity in a white region thatdeviates further to the yellow side than the C region, by means of PWMcontrol of the current regulation section 33 it is possible to match thechromaticities of the emitted lights thereof in a similar manner as thewhite light emitting apparatus 20 to a chromaticity inside the B region.

Thus, when the chromaticity of light emitted by the first or secondwhite LED is broadly selected with a wide region in the band region 66,a target chromaticity point of a color mixture of the lights emittedfrom the two white LEDs or a predetermined white region may notnecessarily be limited to inside the B region, and the target point orpredetermined white region may be a center part between chromaticitypoints of lights emitted by the two white LEDs, or the chromaticitypoint of the color mixture may move to a blue side or a yellow side withrespect to the B region.

By enabling selection of chromaticities of a wide area outside of aranked white region as emitted light colors of the first or second whiteLED in this manner, the usability of white LEDs is increased much more,and product costs can be decreased.

Second Embodiment

The Second Embodiment relates to PWM control of drive currents thateffectively drives the two white LEDs used in the white light emittingapparatus of the First Embodiment to further increase the luminosity ofthe white light emitting apparatus that mixes the emitted light colorsof the two white LEDs. The present embodiment also relates to a whitelight emitting apparatus that enables selection and use of white LEDs inwhich the chromaticities of emitted lights are distributed over a wideregion by means of PWM control. The emitted light colors of the firstand second white LEDs of the Second Embodiment are described usingchromaticities inside the A region and C region, respectively, similarlyto the First Embodiment.

First, current control that adjusts the chromaticity of a white lightemitting apparatus that includes the two white LEDs is described in acase in which, on the CIE chromaticity diagram, there is a difference inthe distance to the target point PD0 of the chromaticity after colormixing from the respective coordinate points of the chromaticities ofthe lights emitted by the two white LEDs 11 and 12 that are used. Atthis time, it is preferable to select LEDs so that the intensity oflight emitted with a rated current by the first white LED 11 and thesecond white LED 12 is approximately the same. In order to make thelight intensities uniform, the light intensity rank of the LEDs may bedesignated and such LEDs may be acquired from a manufacturer, or theLEDs may be individually driven and selected upon measuring the lightintensities thereof.

For example, as illustrated in the CIE chromaticity diagram in FIG. 10,a chromaticity LA2 of an emitted light color from the first white LED 11is positioned at point PA2 in A region, and a chromaticity of an emittedlight color LC2 from the second white LED 12 is positioned at point PC2in C region. A case will now be described in which the hue of the pointPD0 positioned on a line segment connecting the two points PA2 and PC2and at approximately the center of the B region is adjusted. For thispurpose, it is sufficient to adjust the luminosity of each of theemitted light colors so that a weighted average for the coordinates ofPA2 and PC2 obtained by weighting the luminosity of the emitted lightcolors LA2 and LC2 thereof becomes the coordinate values of PD0. In thepresent embodiment, since a distance ratio between the distance(PA2-PD0) and the distance (PC2-PD0) is 1:2 as illustrated in FIG. 10,the emitted light color of the color mixture from the white lightemitting apparatus is made approximately the chromaticity of the pointPD0 by adjusting the ratio of the luminosity between the emitted lightcolors LA2 and LC2 using the ratio 2:1 that is in inverse proportion tothe distance ratio.

As illustrated in FIG. 10, the foregoing description describes a case inwhich the chromaticity PA2 of the first white LED, the targetchromaticity point PD0 of the color mixture, and the chromaticity PC2 ofthe second white LED are on approximately the same straight line on theCIE chromaticity diagram. However, even if these points are not on thesame straight line, by setting the luminosity of the emitted lightcolors LA2 and LC2 in inverse proportion to the distance ratio betweenthe distance (PA2-PD0) and the distance (PC2-PD0), a color mixture ofthe emitted light colors of both LEDs becomes approximately achromaticity of the predetermined white region (B region) that is afavorable chromaticity.

This method of adjusting the color mixture can be expressed in adifferent way. That is, the luminosity of a white LED that emits lightwhose chromaticity is closer to the target point PD0 of the chromaticityafter mixing colors is made higher than the luminosity of the otherwhite LED that emits light whose chromaticity is at a position that isfarther away from the target point PD0. According to the white lightemitting apparatus of the present embodiment for which LEDs that haveapproximately the same light intensity are selected as the two whiteLEDs, the current regulation section 33 is used to continuously supply aconstant current (not a pulse current) from the current regulatingcircuit 21 illustrated in FIG. 2 as the drive current of the white LED11 that is closer to the target point PD0. In this state, only the drivecurrent of the other white LED 12 is supplied using PWM control, and thepulse width t2 thereof is decided so that a mixture of the colors of thetwo white LEDs matches a target chromaticity by a similar method to theFirst Embodiment (see FIG. 11).

With respect to current control other than the current controlillustrated in FIG. 11, it was confirmed that although the color mixtureis approximately the target chromaticity when the current regulationsection 33 is used to set the duration T to 100 milliseconds, t1 (pulsewidth subjected to PWM control with the drive current of the white LED11) to 90 milliseconds, and t2 to 45 milliseconds, the luminosity ishigher in the case of the current control illustrated in FIG. 11.Naturally, it is also favorable to set t1 to be the same as the durationand continuously drive the white LED 11, and set t2 to be half of t1 anddrive the white LED 12.

The method of setting the pulse width in the above PWM control will nowbe summarized. For both the first and second white LEDs, thechromaticity of light emitted by each LED is previously measured with ameasurement sensor device and the respective distances to a targetchromaticity point PD0 after mixing colors on the CIE chromaticitydiagram is calculated. The drive current of the white LED for which thedistance in question is shorter is supplied using a constant current.Further, when supplying a current using PWM control, the setting is madeso that the duty ratio of a pulse width P thereof is the maximum. On theother hand, it is favorable to set a pulse width p for driving the whiteLED for which the distance in question is longer to a value that isinversely proportional to the distance ratio with respect to the pulsewidth P. Naturally, a configuration may also be adopted as in the FirstEmbodiment in which the pulse width that drives the white LED for whichthe distance to the target chromaticity point is longer is adjustedwhile monitoring the color mixture with a measurement sensor device.

According to the present embodiment, a ratio (t1/t2) between the pulsewidth t1 of the white LED 11 that is nearer to the point PD0 and thepulse width t2 of the white LED 12 that is farther from the point PD0can be set to be larger because t1 is a maximum value. This enables thedistance (PC2-PD0) to be made longer with respect to the distance(PA2-PD0), and makes it possible to utilize a white LED that emits lightwhose chromaticity is further from the target point PD0 of the colormixture. This enhances the usability of white LEDs that emit light whosechromaticity deviates from the predetermined white region (B region),and can lower the total cost when acquiring white LEDs.

Third Embodiment

The Third Embodiment relates to a line illuminator 50 that uses thewhite light emitting apparatus 20 of the First Embodiment. The lineilluminator 50 is described in detail below with reference to FIGS. 12to 15.

The line illuminator 50 of the present embodiment is used to illuminatean original copy surface such as a paper surface in an image readingapparatus, for example. The line illuminator 50 is, as illustrated inFIG. 12, provided with a bar-shaped light guide member 51 that is formedof a clear material and has a light incident surface 54 at one endthereof, and a light source section 10 disposed toward the lightincident surface 54. The light source section 10 is connected with thecurrent regulation section 33 via a terminal lead wire 62 as in the caseof the First Embodiment (not illustrated in FIG. 12). The light guidemember 51 is provided with a light guiding section 52 for guiding anincident light from the light incident surface 54 in the longitudinaldirection while causing the light to be reflected by the inner surfaceof the light guide member 51, and a light emitting section 53 having alight emitting surface for causing light from the light guiding section52 to be emitted linearly in the longitudinal direction.

In order that the light from the light source section 10 can be incidentfrom the light incident surface 54 of the light guide member 51 withoutwasting the light, as illustrated in FIG. 13, the external dimensions ofa radiating surface 63 that radiates light of the light source section10 are designed so that the radiating surface 63 can be included insidethe external shape of the light incident surface 54 of the light guidemember 51 with a sufficient margin. For example, on the side of asurface that radiates light of the light source section 10, asillustrated in FIG. 13, two white LED are arranged that each have a sizeof 1.2 mm×2 mm, and the external dimensions of the side of the surfacethat radiates light of the light source section 10 are 2.5 mm(horizontal direction)×2 mm (vertical direction). Further, the externalsize of the light incident surface 54 of the light guide member 51illustrated in FIG. 12 is 3.5 mm (W direction)×2.5 mm (H direction).Furthermore, the design is one in which, by disposing the radiatingsurface 63 of the light source section 10 as near as possible to thelight incident surface 54 of the light guide member 51, light from thewhite light emitting apparatus 20 is made incident on the light guidemember 51 without waste.

The light guide member 51 may be, for example, a light guide member thatis compatible with use for a three-primary color light source that hasLEDs of three wavelengths (for example, for red, green, and blue)arranged thereon (at different positions). That means a light guidemember designed for linear illumination may be used in which lights froma plurality of light sources that have different wavelengths areincident from a light incident surface, and proper reflection andscattering occur in the light guide member for each wavelength, so thata light is emitted with the outputs of the wavelengths being uniformlydistributed in the longitudinal direction thereof. A light guide memberhaving such a function is described in detail in, for example, PatentDocument 1.

Therefore, even when there is a difference in the hues of emitted lightcolors between the two white LEDs 11 and 12 of the light source section10 in the white light emitting apparatus 20 which are used as lightsources, and also the central points of the two emitted lights are notat the same position, and furthermore even when a distance to the lightincident surface 54 from the white light emitting apparatus 20 is shortand sufficient color mixing cannot be obtained in this space, the lightguide member 51 allows the colors of the incident lights from the lightincident surface 54 to be well mixed so as to emit a linear illuminationlight that has a uniform color distribution of whiteness.

The inventors of the present invention checked the above describedeffect of the line illuminator 50 of the present embodiment in thefollowing procedure. First, as illustrated in FIG. 14, the lineilluminator 50 was incorporated in a contact-type image sensor unit(hereinafter, simply referred to as CIS unit) 60 that constitutes animage reading apparatus. Although not illustrated in FIG. 14, thecurrent regulation section 33 of the white light emitting apparatus 20was connected via a connector 61.

The CIS unit 60 was used to cause a light reflected by an original papercopy 59 to be focused on a line sensor 56 by a lens array 55. The linesensor 56 that was used was configured with three linear rows of pixelsthat separately receive a color of red (R), green (G), or blue (B) forphotoelectric conversion (illustration is omitted). The line sensor 56has three color filters that have pass bands for RGB and are disposed oneach row of pixels. Therefore, each row of pixels functions withspectral sensitivity corresponding to each of the R, B, and G colors.Such a sensor array is described, for example, in Patent Document 3.

Therefore, the CIS unit 60 is able to separate the white light reflectedby the original paper copy 59 into each of the R, G, and B colors, andmeasure the illuminance for each pixel of the row of pixels arranged inthe longitudinal direction thereof. The measured illuminance value foreach pixel can be represented as the illuminance distribution for thearea from one end surface on the light incident surface side to theother end surface in the longitudinal direction of the light guidemember 51.

Next, similarly to the First Embodiment, after the original paper copy59 was replaced with a predetermined white paper for reference, both ofthe white LEDs 11 and 12 were driven with the current regulation section33. Further, the illumination light of the line illuminator 50 wasmeasured for the relative illuminance of each of the R, G, and B colorsas the illuminance distribution in the linear direction. In themeasurement, the currents applied to both of the white LEDs 11 and 12were set to 10 mA, respectively. The measured result for illuminancedistributions showed generally uniform distributions in the longitudinaldirection with approximately the same values of relative illuminance forthe red and green colors as illustrated in FIG. 15. Further, for theblue color, the result showed a substantially uniform distribution inthe longitudinal direction, with a higher relative illuminance comparedto those of the red and green colors.

The emitted light color of the white light emitting apparatus 20 thatuses the line illuminator 50 of the Third Embodiment is a light sourcethat has been adjusted so as to have a chromaticity in B region in FIG.7A according to the First Embodiment, and for which the relativeilluminance of blue is somewhat larger. However, differences between thesizes of the relative illuminance of red, green, and blue can beadjusted with a data processing section in the image reading apparatusthat controls the CIS unit. The advantage of a line illuminator thatcaptures emitted light from the white light emitting apparatus 20 of theFirst Embodiment in the light guide member 51 and linearly illuminates apaper surface is that emitted light colors with different hues from twowhite LEDs are adequately mixed in the light guide member 51. A drawbackof a white LED that includes a blue LED chip and a YAG phosphor is thatthere is an inconsistency between the color at the center of the lightbeam and the color at the periphery thereof, such as a strong yellowcolor at the periphery of a beam of emitted light. FIG. 15 illustratesoutput obtained when the respective emitted light colors from the firstand second white LEDs are sufficiently scattered and mixed inside thelight guide member 51 and then used to illuminate an original copysurface with a linear light beam, and the reflected light thereof isreceived with the line sensor 56 via the lens array 55 and subjected tophotoelectric conversion. As illustrated in FIG. 15, although there is adifference between the relative illuminance of each of the R, G, and Bcolors (the relative illuminance of blue is large), it was confirmedthat reflected light from the original copy undergoes color mixingwithout any inconsistency for all of the colors in the main scanningdirection (opposite surface side from light incident surface side) ofthe CIS unit and the relative illuminance is approximately constant andhomogeneous. Accordingly, the line illuminator of the present embodimentis an invention that effectively makes use of the characteristics of thewhite light emitting apparatus 20 of the First Embodiment. Further, aline illuminator that achieves similar advantages can be obtained usingthe white light emitting apparatus of the Second Embodiment.

Fourth Embodiment

A white light emitting apparatus according to a fourth embodiment isprovided with two or more white light emitting diodes. Each of the whitelight emitting diodes includes a blue LED chip, and a fluorescent layerwhich is excited by a radiation light emitted by the blue LED chip andemits a yellow light. Yellow is a complementary color of blue. In thefourth embodiment, a white light emitting apparatus 120 having the twoor more white light emitting diodes will be explained below withreference to FIGS. 17 to 21.

FIG. 16 illustrates a light source section 110 of the white lightemitting apparatus 120 of the fourth embodiment. In the white lightemitting apparatus 120, a first white light emitting diode 111 and asecond white light emitting diode 112 are adjacently mounted to aprinted circuit board 115. The first white light emitting diode 111emits a white light which is more yellowish and deviated from the whitepoint on a chromaticity diagram. The second white light emitting diode112 emits a white light which is more bluish and deviated from the whitepoint on a chromaticity diagram. The white light emitting diodes 111 and112 are also arranged to emit light in parallel to each other and in thesame direction substantially. The printed circuit board 115 is providedwith wiring 116 for electric supply, including an anode line which iscommon to the two white light emitting diodes 111 and 112, and twocathode lines connected to each of the white light emitting diodes 111and 112. The anode line and the two cathode lines are connected to anexternal current control section (see FIG. 18) through terminals A, K1,and K2 respectively. The current control section causes the white lightemitting diodes 111 and 112, which constitute the light source section110 of the white light emitting apparatus 120, to be driven.

The first white light emitting diode 111 may be, for example, a whitelight emitting diode that is recognized to emit a yellowish white lightin accordance with a recognition method by lighting a constant currentof 10 mA among the white light emitting diodes of white LEDs each ofwhich is a commercially available surface mount LED package having alongitudinal dimension of about 2.0 mm and a lateral dimension of about1.2 mm (NESW007A, manufactured by Nichia Corporation). The second whitelight emitting diode 112 may be, for example, a white light emittingdiode that is recognized to emit a bluish white light in accordance witha recognition method by lighting a constant current of 10 mA among thewhite light emitting diodes of white LEDs each of which is acommercially available surface mount LED package having a longitudinaldimension of about 2.0 mm and a lateral dimension of about 1.2 mm(NESW007A, manufactured by Nichia Corporation).

Now, the relationship between chromaticity of the lights emitted by eachof the white light emitting diodes 111 and 112 of the fourth embodimentwill be explained below using the chromaticity diagrams of FIG. 19 andFIG. 30. For the first white light emitting diode 111, a white lightemitting diode is selected so that a blue light, which is emitted from ablue LED chip therein and does not interact with fluorescent particlesin a fluorescent layer, and a yellow light, which is generated bywavelength conversion after the interaction with the fluorescentparticles, produce a color mixture that has its color distribution onthe yellow side of the white point. For the second white light emittingdiode 112, similarly, a white light emitting diode is selected so that ablue light and a yellow light produce a color mixture that has its colordistribution on the blue side of the white point.

The above relationship will be explained below using the chromaticitydiagram illustrated in FIG. 19. In the chromaticity diagram, thechromaticity point of a yellow light (560 nm) after wavelengthconversion by a YAG fluorescent substance is shown by Y, and thechromaticity point of a blue light emitted from a blue LED chip (onerepresentative from the range of 450 nm to 470 nm) is shown by B. When acurved line YB is defined on the assumption that both of thechromaticity points Y and B are positioned near a curved line ST, whichillustrates a monochromatic light in the chromaticity diagram, thecurved line YB passes near the white color chromaticity point W (x=0.33,y=0.33). This is because the YAG fluorescent substance is used to obtaina fluorescent white light by compositing a light emitted from a blue LEDchip and a light emitted by the fluorescent substance.

The chromaticity point WY1 of the color mixture of the light emittedfrom the first white light emitting diode 111 is substantiallypositioned on the curved line WY, which is connected between the whitepoint W and the yellow point Y. The chromaticity point WB1 of the colormixture of the light emitted from the second white light emitting diode112 is substantially positioned on the curved line WB, which isconnected between the white point W and the blue point B.

Meanwhile, one example of chromaticity areas that can be selected withluminescence thereof in a selection using a constant current of 10 mA bya selector is illustrated in FIG. 30. Therefore, as the first whitelight emitting diode 111, a light emitting diode that has a chromaticitycoordinate: 0.33<Cx≦0.36; 0.33<Cy≦0.38 when driven at a constant currentof 10 mA is preferably used, with the chromaticity coordinate beingsubstantially on the curved line YB. Also, as the second white lightemitting diode 112, a light emitting diode that generally has achromaticity coordinate: 0.27≦Cx<0.33; 0.26≦Cy<0.33 when driven at aconstant current of 10 mA is preferably used, with the chromaticitycoordinate being substantially on the curved line YB. As describedabove, in the present invention, the yellowish white means white colorthe position of which is substantially close to the white point on thecurved line WY, while the bluish white means white color the position ofwhich is substantially close to the white point on the curved line WB.

FIG. 18 is a diagram illustrating one example of an electrical circuitdriving the white light emitting apparatus 120. The electrical circuitincludes the light source section 110 and a current control section 133.The light source section 110 corresponds to the part illustrated in FIG.17. The current control section 133 includes current control sectionsthat are separately provided to set a current for each of the whitelight emitting diodes 111 and 112. For example, current regulatingcircuits 121 and 122 are connected in parallel to each of the whitelight emitting diodes 111 and 112 via two cathode terminals K1 and K2 ofthe light source section 110, respectively. Also, transistors T1 and T2turning ON/OFF each of the white light emitting diodes 111 and 112 areconnected to the current regulating circuits 121 and 122, respectively.The transistors T1 and T2 are connected to ground GND.

Each of the current regulating circuits 121 and 122 are, for example,provided with an operation amplifier, a transistor, and current-limitingresistor R1 or R2. The current control section 133 causes a constantcurrent that is defined for each of the white light emitting diodes 111and 112 to be applied to each of the white light emitting diodes 111 and112, so as to control the lighting of each of the white light emittingdiodes 111 and 112 by a pulse width modulation (hereinafter, simplyreferred to as PWM) method. Such current control section 133 functionsas a current control means.

Next, the current control operation for obtaining a color mixture ofemitted lights from the white light emitting apparatus 120 of the fourthembodiment that is substantially adjusted to the white point on achromaticity diagram will be explained below. FIG. 20 is a timing chartillustrating a control of the illumination durations of the white lightemitting diodes 111 and 112 by a pulse width modulation (PWM) method.

First, the white light emitting apparatus 120 is operated with aduration of pulse T being set to be 10 milliseconds, a current drivingthe white light emitting diodes 111 and 112 being set to be 10 mA, anillumination duration t1 of the white light emitting diode 111 being setto be 9 milliseconds for each cycle. The drive currents of the whitelight emitting diodes 111 and 112 are set by the current regulatingcircuits 121 and 122 respectively, and the illumination duration t1 iscontrolled by the transistor T1.

Next, a sensor measuring chromaticity is installed at a position, abovethe light emitting surface of the white light emitting apparatus 120,which is separated from the two white light emitting diodes 111 and 112by a distance so that the emitted lights are sufficiently mixed witheach other, and a measurement of chromaticity is started, with a timefor receiving time being set to be several tens times that of theduration T.

Then, an illumination duration t2 of the white light emitting diode 112for each cycle is controlled by the transistor T2, to find anillumination duration t2 for a chromaticity measurement by a lightemitted from the white light emitting apparatus 120: Cx=0.33, andCy=0.33, approximately.

Here, in order to allow a color mixture of emitted lights from the whitelight emitting apparatus 120 to substantially reach the white point on achromaticity diagram, the PWM control by the current control section 133may be used to cause the white light emitting diode 111 to light inaccordance with a duty ratio D1=t1/T, and the white light emitting diode112 to light in accordance with a duty ratio D2=t2/T.

As described above, according to the fourth embodiment, an emission of alight having a high purity of whiteness, which was difficult by onewhite light emitting diode, is achieved, and the high power white lightemitting apparatus 120 is attained. Also, depending on the control of adrive current of each of the white light emitting diodes 111 and 112,the chromaticity coordinate of a light emission from the white lightemitting apparatus 120 can be changed from the chromaticity point WY1 tothe chromaticity point WB1 along the curved line YB.

As the current control in the white light emitting apparatus 120,instead of the control with the PWM method, the amounts of currentsdriving the white light emitting diodes 111 and 112 may be controlled.The amounts of currents can be controlled by the current regulatingcircuits 121 and 122. Alternatively, the control with the PWM method andthe control of the current amounts may be combined.

Next, a modified embodiment of the fourth embodiment will be explainedbelow with reference to FIG. 21. A white light emitting apparatus 124according to the modified embodiment includes two types of white lightemitting diodes 117 and 118 adjacently mounted on the printed circuitboard 115, the diodes 117 and 118 being individually provided with afluorescent layer 114 of a different thickness from each other. Thatmeans the first white light emitting diode 117 is provided with athicker fluorescent layer 114, and the color mixture of the lightsemitted from the white light emitting diode 117 is yellowish white,similar to the white light emitting diode 111. The second white lightemitting diode 118 is provided with a thinner fluorescent layer 114, andthe color mixture of the lights emitted from the white light emittingdiode 118 is bluish white, similar to the white light emitting diode112. The thickness of the fluorescent layer 114 is controlled during themanufacture of the white light emitting diodes 117 and 118, for example.

The white light emitting apparatus 124 in which the current controlsection 133 is connected to the light source section 110 including theabove described white light emitting diodes 117 and 118 therein can alsoprovide high purity whiteness as in the case of the fourth embodiment.In addition, as compared with the white light emitting diode with oneblue LED chip, a higher power emission of white light can be obtained.

Fifth Embodiment

A linear illuminator 150 according to a fifth embodiment uses the whitelight emitting apparatus 120 according to the fourth embodiment. Thelinear illuminator 150 will be explained in detail below with referenceto FIGS. 22 to 24.

The linear illuminator 150 of the fifth embodiment is used to illuminatea surface of a manuscript such as a paper in an image reading apparatus,for example. The linear illuminator 150 is, as illustrated in FIG. 22,provided with a bar-shaped light guide member 151 that is formed of atransparent material and has a light incident surface 154 at one endthereof, and the light source section 110 disposed toward the lightincident surface 154. The light source section 110 is connected with thecurrent control section 133 via terminal lead 162 as in the case of thefourth embodiment (not illustrated in FIG. 22). The light guide member151 is provided with a light guiding section 152 guiding an incidentlight from the light incident surface 154 in the longitudinal directionof the light guide member, and a light emitting section 153 linearlyemitting the light from the light guiding section 152 in thelongitudinal direction.

In order to improve the light yield of the light guide member 151 fromthe light incident surface 154 into the light guide member 151, thelight source section 110 is designed to have a light emitting surface ofa size that can be included in the light incident surface 154 withmargin. For example, in the case with the light emitting surface of thelight source section 110 having a size of 2.5 mm (horizontaldirection)×2 mm (vertical direction), the light guide member 151 isdesigned to have a light emitting surface having a size of 3.5 mm(horizontal direction)×2.5 mm (vertical direction)

The light guide member 151 may be, for example, a member for a lightsource that has light emitting diodes of three wavelengths (for example,for red, green, and blue) arranged thereon (at different positions).That means a light guide member designed for linear illumination may beused, in which lights from a light source are incident to a lightincident surface, and proper reflection and scattering occur in thelight guide member for each wavelength, so that a light is emitted withthe outputs of the wavelengths being uniformly distributed in thelongitudinal direction thereof. A light guide member having such afunction is described in detail in Japanese Laid-Open Patent PublicationNo. 2006-287923, for example.

Therefore, even when there is a difference in the wavelengths of emittedlights from the two white light emitting diodes 111 and 112 of the lightsource section 110 in the white light emitting apparatus 120, which areused as light sources, and also the central points of the two emittedlights are not at the same position, the light guide member 151 allowsthe colors of the incident lights from the light incident surface 154 tobe well mixed so as to emit a linear illumination light that has auniform color distribution of whiteness.

The inventors of the present invention checked the above describedeffect of the linear illuminator 150 of the fifth embodiment in thefollowing procedure. First, as illustrated in FIG. 23, the linearilluminator 150 was incorporated in a contact image sensor unit(hereinafter, simply referred to as CIS unit) 160 that constitutes animage reading apparatus. Not illustrated, but the current controlsection 133 of the white light emitting apparatus 120 was connected viathe connector 161.

The CIS unit 160 was used to cause a light reflected by the papermanuscript 159 to be focused on a line sensor 156 by a lens array 155.The line sensor 156 was the one configured with three linear rows ofpixels that separately receive a color of red (R), green (G), or blue(B) for photoelectric conversion (illustration is omitted). The linesensor 156 has three color filters that have pass bands for RGB and aredisposed on each row of pixels. Therefore, each row of pixels functionswith spectral sensitivity corresponding to each of the R, B, and Gcolors. Such sensor array is described in Japanese Patent No. 3990437,for example.

Therefore, the CIS unit 160 is able to separate the white lightreflected by the paper manuscript 159 into each of the R, B, and Gcolors, and measure the illuminance for each pixel of the row of pixelsarranged in the longitudinal direction thereof. The measured illuminancevalue for each pixel can be represented as illuminance distribution forthe area from one end surface on the light incident surface side to theother end surface in the longitudinal direction of the light guidemember 151.

Next, as in the case of the fourth embodiment, after the papermanuscript 159 was replaced with a predetermined white paper forreference, both of the white light emitting diodes 111 and 112 weresimultaneously driven with the current control section 133. And theillumination light of the linear illuminator 150 was measured for therelative illuminance of each of R, G, and B colors as illuminancedistribution in the linear direction (FIG. 24A). In the measurement, thecurrents applied to both of the white light emitting diodes 111 and 112were controlled to be 10 mA. The measured result for illuminancedistributions showed generally uniform distributions in the longitudinaldirection with approximately the same values of relative illuminance forred and green colors as illustrated in FIG. 24A, but for blue color, theresult showed a substantially uniform distribution in the longitudinaldirection with a lower relative illuminance as compared to those of redand green colors.

Then, as in the case of the fourth embodiment, after the illuminationduration for each cycle of the white light emitting diode 112, that isthe duty ratio D2, was controlled, the relative illuminance of each ofR, G, and B colors could be controlled to have substantially the samedistribution, as illustrated in FIG. 8B.

The above result of the fifth embodiment showed that the linearilluminator 150 for illumination with the relative illuminance of eachof R, G, and B colors being well balanced can be manufactured.

In a conventional linear illuminator that uses one white light emittingdiode as a light source, often a white light emitting diode for purewhiteness is selected with efforts by sacrificing cost, or a white lightemitting diode having deviation in color distribution of whiteness isselected to obtain a white illumination light of a low quality as aresult. To the contrary, according to the linear illuminator 150 of thefifth embodiment, it was found that when commercially available whitelight emitting diodes having deviation in color distribution ofwhiteness are combined and the illumination duration is controlled bycontrolling a duty ratio by the PWM method, for example, a highly purewhite illumination light can be readily obtained.

The combination of the white light emitting diodes 111 and 112 may beaccomplished by selecting a diode emitting a yellowish white light and adiode emitting a bluish white light, and the chromaticity value of eachdiode which is inherent property of a white light emitting diode may notbe determined at the point of time of selection. Therefore, the whitelight emitting diodes 111 and 112 may be any diode that emits a lightthat has a position generally on the curved line YB in the chromaticitydiagram of FIG. 19, even if the position is deviated from the whitepoint to a large degree. As a result, white light emitting diodes thathave been determined to be defective become usable without discarding,which leads to enhanced productivity of white light emitting diodes.

Sixth Embodiment

In a sixth embodiment, two blue LED chips are mounted to one package,and a YAG fluorescent layer covers each of the blue LED chips and has athickness different from those of others. With reference to FIGS. 25Aand 25B, a method for manufacturing a white light emitting diode 171 ofthe sixth embodiment will be explained below.

A package 172 is formed of a resin into a box shape with the top thereofbeing open, and the package 172 has wiring of a lead frame at the bottomthereof, so that an anode wiring 175 of a lead frame at the bottom has afirst blue LED chip 173 and a second blue LED chip 174 disposed thereon.Then, the cathode terminal and the anode terminal thereof are connectedto the cathode wiring 176 and the anode wiring 175 of the lead framerespectively by using a wire-bonding method for example, to be mounted(FIG. 25A). The top opening may have a dimension of about 2.5 mm or lessin the longitudinal and lateral directions thereof, for example.

Next, a resin solution for fluorescent layer is prepared by mixing apredetermined amount of YAG fluorescent particles into a clearthermosetting transparent resin, which is coated to both of the blue LEDchips 173 and 174 for covering. Then, the resin solution for fluorescentlayer is subjected to a heat cure procedure, so that a first fluorescentlayer 178 is formed as illustrated in FIG. 25B. Then, a resin solutionfor fluorescent layer which is the same as the above resin solution forfluorescent layer is coated to the surface of the fluorescent layer 178to cover only the upper side of the blue LED chip 173. Then, the resinsolution for fluorescent layer is subjected to a heat cure procedure, sothat a second fluorescent layer 179 is formed. Then, a seal body 180 isformed on the fluorescent layers 178 and 179 using the samethermosetting transparent resin as that included in the resin solutionfor fluorescent layer. In this way, as illustrated in FIG. 25B, thewhite light emitting diode 171 is made.

The thicknesses of the fluorescent layers 178 and 179 may be determinedin advance as follows, for example. First, a plurality of blue LED chipsthat are the same products as the blue LED chips 173 and 174 areprovided, and fluorescent layers having different thicknesses from eachother are formed using the resin solution for fluorescent layer that isprepared as described above. As a result, a plurality of white lightemitting diodes for evaluation can be obtained. Next, the white lightemitting diodes for evaluation are caused to emit a light at apredetermined current, so that the diodes that produce a color mixtureof bluish white without fail with the blue light, which does notinteract with the fluorescent particles, and the yellow light, whichinteracts with the fluorescent particles in the fluorescent layer forwavelength conversion, and a range of the thickness of the fluorescentlayer 178 is determined based on the thicknesses of the fluorescentlayers. Next, a plurality of white light emitting diode that has thefluorescent layer 178 having a thickness within the range formed on eachblue LED chip are provided, and a fluorescent layer is formed on eachdiode with the resin solution for fluorescent layer that is prepared asdescribed above. As a result, a plurality of white light emitting diodesfor evaluation can be newly obtained. Next, each of the white lightemitting diodes for evaluation is caused to emit a light at apredetermined current, so that the diodes that produce a color mixtureof yellowish white without fail with the blue light, which does notinteract with the fluorescent particles, and the yellow light, whichinteracts with the fluorescent particles in the fluorescent layer forwavelength conversion, and based on the thicknesses of the fluorescentlayers, a range of the thickness of the fluorescent layer 179 isdetermined.

The white light emitting diode 171 made as described above may be usedby mounting to a printed circuit board 182 as illustrated in FIG. 26,for example. The light emitting diode 171 may be driven by the currentcontrol section 133 of the fourth embodiment, for example, and isindividually connected to a lead wirings a of the anode wiring 175 andthe lead wirings k1, k2 of the two cathode wirings 176. In the case, thewhite light emitting apparatus having the white light emitting diode 171has an electrical circuit configuration that is equal to that having theblue LED chips 173 and 174 instead of the white light emitting diodes111 and 112 of FIG. 18.

The degree of whiteness of the light emitted from the white lightemitting diode 171 made as described above may be controlled as follows.First, the current control section 133 is used to set the current valuewhich is applied to the blue LED chips 173 and 174 to be 20 mA, and thePWM method similar to that in the fourth embodiment is used to causeboth of the blue LED chips 173 and 174 to be driven. Then, theillumination duration of each of the blue LED chips 173 and 174 for eachcycle is controlled to determine a duty ratio to drive each of the blueLED chips 173 and 174 so that the light emitted from the white lightemitting diode 171 has an average wavelength distribution (the colormixture) generally at the white point.

The relationship can be explained with reference to the chromaticitydiagram of FIG. 27 as follows. The color mixture of the light emittedfrom the white light emitting diode 171 when only the blue LED chip 173is lit by applying a current of 20 mA is detected as a chromaticitypoint WY3, which is substantially positioned on the curved line WY.Similarly, the color mixture of the light emitted from the white lightemitting diode 171 when only the blue LED chip 174 is lit by applying acurrent of 20 mA is detected as a chromaticity point WB3, which issubstantially positioned on the curved line NB. This is because thethicknesses of the fluorescent layers 178 and 179 are adequatelydefined. The coordinates of the chromaticity points WY3 and WB3 on thechromaticity diagram are (Cx=0.36 or more, Cy=0.39 or more), and(Cx=0.26 or less, Cy=0.25 or less), for example, respectively.Therefore, the chromaticity coordinates may be outside the chromaticityareas a0, b1, b2, and c0 in FIG. 30 that are allowed by selection.

Next, the duty ratios of both of the blue LED chips 173 and 174 in a PWMmethod driving are individually controlled, and the illuminationduration is detected which results in a chromaticity measurementapproximately equal to the whiteness point (Cx=0.33, Cy=0.33) for thecolor mixture of the light emitted from the white light emitting diode171.

As described above, the white light emitting diode 171 of the sixthembodiment includes two blue LED chips 173 and 174 therein, and has YAGfluorescent layers 178 and 179 that cover the chips 173 and 174, andhave adequately defined thicknesses. And the control of the duty ratiofor lighting the blue LED chip 173, which is covered with both of thefluorescent layers 178 and 179, allows the amount of the emittedyellowish white light to be controlled. Also, the control of the dutyratio for lighting the blue LED chip 174, which is covered with only thefluorescent layer 178, allows the amount of the emitted bluish whitelight to be controlled. The controls make the color mixture of the lightemitted from the white light emitting diode 171 fall on the white pointW.

As described above, according to the white light emitting diode 171 ofthe sixth embodiment, a white light emission can be obtained withoutspecial caring about the variation in the inherent wavelengths of thelight emitted from the blue LED chip, the variation in the yellowwavelength due to the formation of the fluorescent layers, and the like.Furthermore, the light emission from the white light emitting diode 171can be readily controlled to be enhanced to a high-quality white lightemission. In addition, as compared with the white light emitting diodewith one blue LED chip, a higher power emission of white light can beobtained.

In the sixth embodiment, the light emission of the white light emittingdiode 171 when only the blue LED chip 173 is driven at 20 mA is, asillustrated in FIG. 27, represented by the chromaticity point WY3 foryellowish white without fail. Similarly, the light emission of the whitelight emitting diode 171 when only the blue LED chip 174 is driven is,as illustrated in FIG. 27, represented by the chromaticity point WB3 forbluish white without fail. When the thicknesses of the fluorescentlayers 178 and 179 are adequately determined and reliable yellow andblue colors are obtained, the chromaticity points WY3 and WB3 may be setat the positions separated from the chromaticity point WY1 for“yellowish white” and the chromaticity point WB1 for “bluish white” inthe fourth embodiment respectively (FIG. 19) based on the chromaticitypoint W for white color as a reference. This is because the control ofthe drive currents that are applied to each of the blue LED chips 173and 174 allows the chromaticity of the light emitted from white lightemitting diode 171 also to reach the white point W as in the case of thefourth embodiment. Furthermore, according to the white light emittingdiode 171 of the sixth embodiment, the chromaticity of emitted light canbe controlled within a range from the chromaticity point WY3 to thechromaticity point WB3 along the curved line YB, which is wider thanthat of the fourth embodiment.

In the sixth embodiment, the ratio of wavelength conversion of the bluelights from the blue LED chips 173 and 174 to yellow lights iscontrolled using the thicknesses of the fluorescent layers 178 and 179,but the ratio may be controlled by the concentration of YAG fluorescentparticles dispersed in the fluorescent layers 178 and 179. Also, in thesixth embodiment, the amounts of the emitted yellowish light and theemitted bluish light are controlled using the illumination duration perpulse, but may be controlled using the current values applied to theblue LED chips 173 and 174.

Seventh Embodiment

A linear illuminator 190 according to a seventh embodiment uses thewhite light emitting diode 171 according to the sixth embodiment. Thelinear illuminator 190 will be explained below in detail with referenceto FIGS. 28 and 29.

The linear illuminator 190 of the seventh embodiment is also used toilluminate a surface of a manuscript such as a paper in an image readingapparatus, for example. The linear illuminator 190 is configured with aprinted circuit board 182 to which the white light emitting diode 171according to the sixth embodiment is mounted, instead of the lightsource section 110 in the fifth embodiment, as illustrated in FIG. 28.

The linear illuminator 190 incorporated in a CIS unit 191 provides theimage reading apparatus illustrated in FIG. 29. Not illustrated, butsimilar to the fifth embodiment, the current control section 133 isconnected via the connector 161. Other configurations are similar tothose in the fifth embodiment.

Therefore, even in the case where there is a difference in wavelengthsof emitted lights between the blue LED chips 173 and 174 in the whitelight emitting diode 171 used as a light source and the two centralpoints of the emitted lights are not at the same position, the lightguide member 151 is able to emit a linear illumination light ofuniformly distributed color of whiteness after sufficient color mixtureof incident lights from the light incident surface 154.

The inventors of the present invention checked the above describedeffect of the linear illuminator 190 of the seventh embodiment in thefollowing procedure. As in the case of the fifth embodiment, the linearilluminator 190 was incorporated in the CIS unit 191 that constitutes animage reading apparatus.

Next, as in the case of the fifth embodiment, after the paper manuscript159 was replaced with a predetermined white paper for reference, theblue LED chips 173 and 174 with the current control section 133 weresimultaneously driven at a current value of 20 mA using the PWM method.Then the relative illuminance of each of R, G, and B colors and theilluminance distribution in the linear direction were measured.

Then, similar to the fifth embodiment, based on the measured result,each of the duty ratios of the current pulses for driving the blue LEDchips 173 and 174 were adjusted, as the result of that the relativeilluminance of each of R, G, and B colors could be adjusted to begenerally uniformly distributed across the entire width of the originalpaper copy 159.

The above result of the present seventh embodiment showed themanufacture of the linear illuminator 190 can be achieved that emits alight with the relative illuminance of each of R, G, and B colors beingwell balanced using the white light emitting diode 171.

INDUSTRIAL APPLICABILITY

The white light emitting apparatus and line illuminator according to thepresent invention are advantageously utilized in an image readingapparatus that supports reading of color images such as an imagescanner, a facsimile machine, or a copier.

1.-19. (canceled)
 20. A white light emitting apparatus for an imagereading apparatus, that uses a white LED having a blue LED chip and aphosphor, comprising: a light source section in which a first white LEDthat emits white light at a first chromaticity point of firstchromaticity coordinates, the first chromaticity point deviating to ablue side from a predetermined white region of a CIE chromaticitydiagram, and a second white LED that emits white light at a secondchromaticity point of second chromaticity coordinates, the secondchromaticity point deviating to a yellow side from the predeterminedwhite region, are adjacently disposed such that optical axes thereof arein substantially the same direction; and a current regulator thatindependently drives the blue LED chip of the first and second whiteLEDs, respectively; wherein the current regulator performs PWM controlfor a drive current of at least one of the first and second white LEDs,and by setting respective pulse widths for driving at least one of thefirst and second white LEDs that are inversely proportional to adistance from a target chromaticity point to a chromaticity point ofemitted light of the first white LED and a distance from the targetchromaticity point to a chromaticity point of emitted light of thesecond white LED on a CIE chromaticity diagram, adjusts a color mixtureof lights emitted from the first and second white LEDs to a chromaticityof the predetermined white region.
 21. A white light emitting apparatusfor an image reading apparatus, that uses a white LED having a blue LEDchip and a phosphor, comprising: a light source section in which a firstwhite LED that emits white light at a first chromaticity point of firstchromaticity coordinates, the first chromaticity point deviating to ablue side from a predetermined white region of a CIE chromaticitydiagram, and a second white LED that emits white light at a secondchromaticity point of second chromaticity coordinates, the secondchromaticity point deviating to a yellow side from the predeterminedwhite region, are adjacently disposed such that optical axes thereof arein substantially the same direction; and a current regulator thatindependently drives the blue LED chip of the first and second whiteLEDs, respectively; wherein, the current regulator: continuouslysupplies a constant current to one white LED among the first and secondwhite LEDs that emits white light whose chromaticity is nearer to atarget chromaticity point on the CIE chromaticity diagram, and suppliesa PWM-controlled current to another white LED among the first and secondwhite LEDs, sets a duty ratio of the PWM control to a value of aninverse ratio between a distance between a chromaticity of the one whiteLED and the target chromaticity point and a distance between achromaticity of the other white LED and the target chromaticity point onthe CIE chromaticity diagram, and adjusts a color mixture of emittedlights from the first and second white LEDs to a chromaticity of thepredetermined white region.
 22. A line illuminator configured to emitlight that is incident from a light source disposed toward a lightincident surface provided at an end surface in a length direction of abar-shaped light guide member formed of a clear member from a lightemitting surface provided along a length direction while causing thelight to be reflected by an inner surface of the barshaped light guidemember, wherein the light source is a white light emitting apparatusaccording to claim
 20. 23. A line illuminator configured to emit lightthat is incident from a light source disposed toward a light incidentsurface provided at an end face in a length direction of a bar-shapedlight guide member formed of a clear member from a light emittingsurface provided along a length direction while causing the light to bereflected by an inner surface of the barshaped light guide member,wherein the light source is a white light emitting apparatus accordingto claim
 21. 24. The line illuminator according to claim 22, wherein anexternal shape of a radiating surface that radiates a light of the whitelight emitting apparatus is of dimensions that can be included in anexternal shape of the light incident surface of the light guide member.25. The line illuminator according to claim 23, wherein an externalshape of a radiating surface that radiates a light of the white lightemitting apparatus is of dimensions that can be included in an externalshape of the light incident surface of the light guide member.
 26. Awhite light emitting apparatus, comprising: a first white light emittingdiode emitting yellowish white; a second white light emitting diodeemitting bluish white in the same direction of emission by the firstwhite light emitting diode; and a current controller controlling drivecurrents of the first and second white light emitting diodes, whereinthe current controller controls duty ratios of outputs of the drivecurrents.
 27. The white light emitting apparatus according to claim 26,wherein the current controller controls values of the drive currents.28. The white light emitting apparatus according to claim 26, whereineach of the first and second white light emitting diodes comprises: ablue light emitting diode chip; and a wavelength conversion layer forwavelength conversion of a part of blue light from the blue lightemitting diode chip into yellow light.
 29. A white light emittingapparatus, comprising: a first white light emitting diode emittingyellowish white; a second white light emitting diode emitting bluishwhite in the same direction of emission by the first white lightemitting diode; and a current controller controlling drive currents ofthe first and second white light emitting diodes, wherein each of thefirst and second white light emitting diodes comprises: a blue lightemitting diode chip; and a wavelength conversion layer for wavelengthconversion of a part of blue light from the blue light emitting diodechip into yellow light, and the wavelength conversion layer in thesecond white light emitting diode has a thickness smaller than that ofthe wavelength conversion layer in the first white light emitting diode.30. A white light emitting diode, comprising: first and second bluelight emitting diode chips; and a wavelength conversion layer forwavelength conversion of a part of blue light from the first and secondblue light emitting diode chips into yellow light, wherein thewavelength conversion layer emits yellowish white after wavelengthconversion of a part of blue light from the first blue light emittingdiode chip into yellow light, and emits bluish white after wavelengthconversion of a part of blue light from the second blue light emittingdiode chip into yellow light, wherein the wavelength conversion layercomprises: a first fluorescent layer covering the first and second bluelight emitting diode chips; and a second fluorescent layer covering onlythe second blue light emitting diode chip of the first and second bluelight emitting diode chips.